The present invention generally relates to microwave feed horns, and more particularly to a compensated microwave feed horn which reduces cross polarized components of circular polarized radiation emanating therefrom.
State of the art gigahertz antenna arrays for radar systems utilize one wavelength diameter feed horns. The diameter of such feed horns is approximately one inch. When driven at their input waveguide ports with circularly polarized energy, such feed horns generate far field radiation patterns which contain undesirable cross polarized, or opposite sense circularly polarized, components which vary in amplitude from negligible on axis to an undesirable level off axis.
Conventional Potter-type feed horns have operating characteristics which would provide for reduced cross polarization components in off axis directions. However, the Potter-type feed horns are typically larger aperture horns and such one wavelength diameter feed horns are difficult to optimize. A better understanding of Potter-type feed horns may be found from a reading of "A New Horn Antenna with Suppressed Sidelobes and Equal Beamwidths," by P. D. Potter, Microwave Journal, Vol. VI, pages 71-78, June 1963, "The Circular Waveguide Step-Discontinuity Mode Transducer," by W. J. English, IEEE Trans. Microwave Theory Tech., Vol. MTT-21, pages 633-636, October 1973, and "Phase Characteristics of a Circularly Symmetric Dual-Mode Transducer," by K. K. Agarwal, IEEE Trans. Microwve Theory Tech., Vol. MTT-18, pages 69-71, January 1970.
However, it has been found that by exciting a one inch diameter conventional feed horn with linearly polarized energy, the far field H plane and E plane radiation patterns have substantially equal magnitudes off axis at approximately 45 degrees, the axial ratio measurements using circularly polarized energy excitation indicate a phase difference off axis between E and H planes.